In a rare step for a poet laureate, Andrew Motion today speaks out in his newest poem against the momentum towards a US-led invasion of Iraq using British forces who would be serving nominally under the Queen.

In the 30-word poem, Motion, who was appointed by the Queen in 1999, sides with those who are "doubtful" about a war - and against the political leaderships of Britain and America.

He said yesterday that the leaders' rhetoric hid "several of the motives which are actually driving the thing forward. In other words, it's as much to do with oil, imperialism and a sort of strange father fixation [on President Bush's part]. They are not being candid".

The poem, printed here exclusively, is called Causa Belli, a Latin phrase translated as "causes, motives or pretexts of war". It is based on an anti thesis between "They... ", the leaders, and "Our straighter talk...", that of doubters in conversations among the public.

In the poem, the doubters' voices are "drowned" by the leaders. But their arguments are also described as "ironclad" because, Motion said yesterday, "they will endure".

Motion's most famous precedent for doing this as poet laureate is Alfred Lord Tennyson. Tennyson included in his poem The Charge of the Light Brigade the controversial and popular lines "... the soldier knew/someone had blundered".

However, he was writing after the Crimean war. During the second world war John Masefield as laureate published two patriotic volumes, A Generation Risen and Some Verses to Some Germans.

Motion added: "I do believe that, if there are weapons of mass destruction discovered in Iraq, something needs to be done.

"There is no compelling evidence yet. It may still come to light, in which case the picture changes. This is not a poem about whether we should go to war. We can't decide that because we don't yet know whether there are weapons. It's a poem about wishing to be more candid.

"I have absolutely no misgivings about getting short words from the Queen. In fact, if weapons do turn out to be there, I may well write a poem supporting going."

During his tenure as laureate, Motion has often departed from the tradition of ceremonial poems on royal occasions. He has written on Nelson Mandela, national identity, homelessness and bullying.

He said: "My underlying feeling is that poetry ought to be part of general life rather than being ghettoised."

CAUSA BELLI by Andrew Motion

They read good books, and quote, but never learn a language other than the scream of rocket-burn. Our straighter talk is drowned but ironclad: elections, money, empire, oil and Dad.